Wherever possible when performing construction work at sea or in a river (offshore platforms, sunken tunnels, under river crossings, bridges, etc. . .), use is made of prefabricated structures which are brought on site while afloat and which are then grounded in their final positions.
It is desirable to reduce the amount of work that must be performed on site to as little as possible, and to increase the amount of work performed by prefabrication, since prefabrication takes place on land or in a dry dock and consequently under better conditions of: cost; time taken; quality of work performed; and safety of personnel performing the work.
However, prefabrication can be of limited use only, or of no use at all, when the positioning tolerances required for the work under construction are incompatible with the tolerances which can be achieved by grounding, i.e. tolerances of one to several meters. This is particularly true for multi-span bridges, in which the positioning tolerances required for the piers, the pylons, and the spans have an order of magnitude of about ten centimeters. In this case, either the entire work is constructed on site, or else prefabrication is limited to a portion of the work with the rest of it then being constructed on the final site. For example a foundation may be prefabricated and grounded to within plus or minus two meters, with a pier then being constructed on the grounded foundation to the required tolerance. However, when this is done, and given the relatively poor accuracy with which the prefabricated foundation can be positioned by grounding, it is sometimes necessary to provide a foundation of sufficiently large size to ensure that after it has been grounded, the bridge pier can subsequently be built on the foundation in a position and at an orientation both of which are accurate relative to other piers of the bridge which have already been constructed. As a result, the prefabricated foundations must be larger in size than would be strictly necessary for receiving a bridge pier. In addition, even if a portion of the work can be prefabricated on land, a large quantity of the work remains to be done on the final site (pier construction,pylon construction or mounting, installing the bridge deck or spans, etc. . .).
There are several causes for poor accuracy in grounding maneuvers (one to several meters). Firstly, it is practically impossible to hold a floating structure totally still while the structure is subjected to the action of swell and of currents. The structure which is to be held still by mooring lines prior to grounding is nevertheless subjected to motion which is more or less periodic with amplitude which is a function of the stiffness of the mooring lines and of the forces due to currents, swell, and wind. The amplitude of such motion may be reduced by shortening the mooring lines (or by increasing their stiffness), but in that case the forces in the mooring lines also increase and a limit is rapidly reached due to the strength of the mooring devices (cables, anchors, mooring blocks, etc.). Further, the lack of a fixed reference point makes position-determining operations difficult and inaccurate, thereby making guidance and positioning prior to grounding difficult and inaccurate.
The main aim of the present invention is thus to provide a method enabling a prefabricated structure to be grounded, and in particular enabling a foundation for a bridge pier to be grounded, with the structure being grounded accurately both in position and in orientation on the sea bed or on a river bed.